The present invention relates to moisture proofing electrical, medium or high voltage cable.
A known variety of electrical cable for medium or high voltages consists of a cable core, an insulating jacket surrounding the core, an electrically conductive coating on the jacket and a shield on the coating, the shield consisting, for example, of wires which have been stranded onto the core-jacket-coating sub-assembly and are held thereon by suitable ribbons. Subsequently, the assembly is completed by extruding an outer protective jacket onto the shield.
Occasionally, the outer jacket is damaged so that moisture may penetrate into the cable. Moisture may seep through any gaps between the shield wires, the ribbons etc. and sooner or later corrosion starts to set in. Also, moisture which has penetrated into the interior of the cable degrades the insulation, and so-called water treeing will sooner or later cause insulation failure and voltage breakdown.
The corrosion mentioned above is usually the result of the formation of electrolytic elements. The shield wires are made of copper or aluminum, and the electrically conductive coating on which the wires sit, contains graphite. Thus, moisture will act as electrolyte for these two electrochemically different parts. The electro-corrosion will destroy the shield or at least damage it locally.
Ingress of moisture will also lead to damage to terminating fittings and connection sleeves of the cable. It can readily be seen further that the various problems outlined above will be particularly pronounced in underwater cable. The cable is subjected to the rather high hydrostatic pressure, forcing water into any leak.
German printed patent application No. 25 14 891 suggests embedding the shield wires in a conductive coating. Corrosion can be avoided to some extent in that manner. This coating serves also as a barrier against further penetration of moisture into the insulation underneath. However, submarine cable as well as underground high voltage cable are not sufficiently protected in that manner.
In the case of underwater cable, the water pressure may well suffice to cause water to penetrate the barrier. A high voltage underground cable has usually thick insulation but sets up also very high local electrical field strength. Moreover, the polymer insulation has a rather high thermal coefficient of expansion. Thus, a thick insulation jacket will expand and contract, also called breathing, so that the shield experiences a rather strong mechanical load, and gaps, cavities or pockets may readily form in the interior of the cable.
Generally speaking, it is also known that the types of plastic used for making cable jackets are not completely impervious in regard to diffusion of moisture. Thus, one has jacketed such cables, particularly in critical cases, in a lead cover or by longitudinally folding an overlapping metal foil around the cable upon which a polymer jacket is extruded. Such a construction provides adequate protection of an undamaged cable under normal operating conditions. However, after the jacket has been damaged, moisture may well migrate into the cable, in axial direction, and do the various kind of damage outlined earlier.